1. Field of the Invention
The subject invention relates to a rain sensor for detecting moisture on the windshield of a vehicle for turning on the windshield wipers, moisture taking the form of mist, water, drops as a film, frost, ice, snow, fog, etc.
2. Description of the Prior Art
Various optical rain-sensing methods have been developed in order to permit fully automatic windshield wiper operation, particularly for automotive vehicles. These range from simple extinction and backscatter sensors, such as those described in U.S. Pat. Nos. 4,798,956 and 4,960,996 to P. A. Hochstein, to more sophisticated imaging sensors, such as those shown in U.S. Pat. No. 5,923,027 to Stam, et al and U.S. Pat. No. 6,144,022 to Tenenbaum, et al.
The problem with extinction sensors is that their field of view or active area is rather small, generally on the order of 500 mm2. Human factor experiments have shown, however, that sensing areas on the order of 104 mm2 are necessary to properly control the windshield wipers in vehicles. The reason for such a relatively large sensing area on the glass is that drivers want a certain level of minimum clear vision. That is, drivers want a given degree of visibility that is a function of ambient conditions and rain profile. Small and closely spaced droplets are generally not viewed as deleterious to safe vision as are a few large drops, which may be spaced at greater distances from one another. On the basis of several careful human factors studies, it has been determined that a sensing area of at least 8000 mm2 was necessary, and a minimum water drop sensitivity of 1.0 mm (diameter) was desirable.
Achieving such relatively large sensing areas with extinction rain sensors is unwieldy and possibly unsafe. All known extinction rain sensors are directly mounted to the glass, and as such would obscure the driver""s vision if they were large enough to be truly useful.
Off the glass rain sensors are more easily adapted to provide a greater field of view of the windshield, and may be mounted away from the glass thereby providing less (or no) obscuration of the glass. Various imaging rain sensors such as the backscatter sensor of the U.S. Pat. No. 5,313,072 to Vachss or the spatial image analysis sensor of U.S. Pat. No. 5,923,027 to Stam, et. al employ lenses and focal plane arrays to permit remote sensing rather than on the glass sensing. The image correlation rain sensor of the aforementioned U.S. Pat. No. 6,144,022 to Tenenbaum, et al and the aforementioned Ser. No. 09/267,867 filed Mar. 12, 1999 (attorney docket 65165.004) also create a sequence of images by means of a lens and a CMOS imager or focal plane detector, both of which allows for remote mounting.
All these sensors do provide the necessary convenience of remote measurement of a relatively large, representative portion of the windshield, but they do so at a high price. Their sensitivity and selectivity to rain is limited. That is, the signal to noise ratio implicit in these imaging rain sensors is low and the respective devices are prone to mis-trigger, false trigger or do not respond to rain in certain operating conditions.
The primary interference to all optical, imaging rain sensors reside in the intense ambient light or specular light sources which fall within the field of view of the sensor; in other words, the existing artifacts and spurious signals in the ambient surroundings interfere with or provide false sensing indicating the false presence of raindrops. Certain accommodations, such as fast low f/member optical systems, can minimize such interference by maximizing the image contrast for objects at or close to the windshield. However, the very large variances in ambient light, typically 10xe2x88x922 Lux to over 104 Lux, intrinsically limit the efficacy of single ended signal processing systems. In other words, not knowing what signal to expect limits the filtering one can apply to the captured image. While certain assumptions can reasonably be made as to the nature of rain drop distribution across the glass, the variety of driving conditions precludes the use of image templates or similarly narrow filtering algorithms. Specifically, it is rapidly changing ambient light or lighted artifacts that confuse these prior art, imaging rain sensors, i.e., create the interference. As an example, the sophisticated sensor shown in the aforementioned U.S. Pat. No. 6,144,022 to Tenenbaum et al. al uses a digital correlation technique to discriminate between raindrops and slowly changing ambient lighting conditions. Under the influence of quickly moving headlights or tail/brake lamps, the subsequent image frames are no longer correlated, thereby assuming a rain condition, i.e., presence of raindrops. Even by defocusing the interfering image by using very low f/number optics, sufficient changing energy from extraneous sources has been found to interfere with sensor operation. The same is true in the aforementioned application Ser. No. 09/267,867 filed Mar. 12, 1999 (attorney docket 65165.004) in that sequential images are taken which include the changing ambient light conditions, which may include constantly changing interfering artifacts and spurious light. The prior art which sometimes uses ancillary illumination to improve rain drop image contrast, does not synchronously pulse the illumination source with the imager (camera) framing interval.
Accordingly, there remains a need for a more precise imaging rain sensor to activate the wipers only in response to a predetermined level of moisture on the windshield and not be affected by the constantly changing interfering artifacts and spurious light signals in the ambient light.
The present invention greatly improves the selectivity of imaging rain sensors to rain while simultaneously rejecting interfering artifacts and spurious signals.
In accordance therewith, the invention provides a method for sensing moisture on the exterior surface of a sheet of glass including the step of capturing an ambient image of light rays passing through the glass from the exterior to the interior surfaces thereof under ambient light conditions. The method is characterized by illuminating the glass with light rays, capturing an illuminated image of the glass containing the ambient image plus reflected light from the illuminating of the glass, and subtracting the ambient image from the illuminated image to produce a moisture signal.
The invention also provides an assembly for sensing moisture on a glass surface from a position adjacent the glass comprising a light source for illuminating the glass with light rays, and an imaging device adapted for disposition adjacent to the interior surface of a sheet of glass for capturing an ambient image of light rays passing through the glass from the exterior to the interior surfaces thereof under ambient light conditions and for capturing an illuminated image of the glass containing the ambient image plus reflected light from the illuminating of the glass. The assembly is characterized by a controller for subtracting the ambient image from the illuminated image to produce a moisture signal.
Synchronous detection, or more precisely, differential image analysis of the present invention yields benefits similar to those (in terms of s/n ratio) that accrue in other (communications) applications. Those benefits are due to the fact that the pulsed illuminant only illuminates the subject of interest, which is water drops on the outer surface of the windshield. Extraneous objects, which lie outside the object plane, are not illuminated to any substantial degree, thereby greatly improving differential rain sensor selectivity.